Polyimide, or more specifically aromatic poly (ether-imide), such as poly (4, 4' oxydiphenylenepyromellitimide) sold under the brand name Kapton, is used widely in electronic and microelectronic fabrication because of its high temperature stability and low dielectric constant. This material, in the form of a film or tape, is used, for example, as the substrates of lead frames or so-called TAB tapes in IC chip packaging as well as the substrates of flexible printed circuits of more general application. The lead frames and circuits are metal-polyimide composites or laminates with the metal being adhered to the substrate or, more preferably, applied directly to the substrate by known sputtering or electroless or electrolytic plating processes as described, for example, in U.S. Pat. No. 4,832,799. The metal which ultimately forms the conductive traces or paths of the lead frame or circuit is usually copper, although there may be an intermediate layer of cobalt or nickel between the copper and the substrate to optimize the bond between the metal and the substrate.
Polyimide has a disadvantage, however, in that it is very permeable to organic solvents and inorganic cations. Accordingly, when the processing of the polyimide based article to its finished form requires exposure to such solvents and ions, the solvents and ions tend to diffuse into and contaminate the polyimide. In many instances, such contamination can degrade the finished article and affect the longterm reliability of that article.
For example, in the manufacture of a lead frame or flexible circuit from a metal-polyimide composite or laminate by a subtractive process, a layer of photoresist is applied to the metal layer and photoimaged through a mask corresponding to the conductive pattern to be formed on the lead frame or circuit. The composite is then subjected to a solvent bath which removes the photoresist except in those areas which received no light corresponding to the desired conductor paths. The thus imaged metal-polyimide composite is then subjected to an etching solution which etches away the unmasked metal thereby forming the lead frame or circuit. Finally, the lead frame or circuit is exposed to another solvent which strips the masking photoresist from the surfaces of the metal conductors. In a similar manner, using known additive processes, metal can be controllably plated onto the substrate to form the conductor pattern.
A commonly used photoresist solvent or wash in the fabrication of lead frames and printed circuits is methylene chloride. However, when that wash is applied to a polyimide substrate, it diffuses into the polyimide. In the presence of moisture and oxygen, e.g. moist air, the methylene chloride in the polyimide can oxidize to hydrochloric acid. That acid can, in turn, cause local corrosion of the metal conductors at the metal-polyimide interface. Such corrosion can result in eventual failure of the circuit, e.g. a short, due to the electrochemical migration of the corrosion products. Extreme corrosion can cause complete delamination of conductor segments from the substrate. Worse still, the effects of such corrosion may not manifest themselves for some time. Such corrosion can be accelerated by a high temperature bake of the circuit for an extended time and the circuit inspected for corrosion. However, this procedure is both costly and time consuming. Further, that inspection bake can itself degrade the strength of the metal-polyimide bond and adversely effect the long term reliability of the lead frame or circuit being inspected.
Contamination of the polyimide can also occur when plating the conductors of such a lead frame or printed circuit. That is, in some instances, the specifications call for a layer of metal such as gold, gold alloy or nickel to be plated onto the copper conductors of the lead frame or circuit. This requires immersion of the circuit in a plating solution which usually contains ions of an alkali metal such as cesium, potassium or sodium to increase the conductivity of the solution. These ions, or more accurately cations, tend to diffuse into the polyimide. During the plating step, electric current flows through the polyimide substrate as well as the copper layer causing electrochemical reduction of the polyimide which results in degradation of the adhesion strength of the bond between the metal conductors and the polyimide substrate.
This problem of solvent and cation contamination of polyimide can be alleviated or avoided by applying a protective coating to the surface of the polyimide thereby isolating the polyimide when those contaminants are encountered during fabrication of a lead frame, printed circuit or other polyimide-based article. However, such coatings are cumbersome and they complicate the fabrication process because the coating first has to be applied to and then removed from the polyimide at particular stages of the process.